Torque applied to a tire through a drive shaft acts to propel a vehicle by the friction between the tire and the surface of the road for the vehicle. At that time, a slip takes place between the road surface and the tire. The ratio of the slip depends on the coefficient of friction between the tire and the road surface. The coefficient of friction fluctuates due to the states of the road surface and the tire, the grounding load upon the tire, the magnitude of the torque transmitted to the tire, the driving speed of the vehicle, and so forth.
As for an ordinary two-wheel-drive vehicle, high torque is transmitted to each driving wheel through a transmission at the start of the vehicle so that a large slip takes place between the road surface and the tire of the wheel. The torque transmitted through the transmission decreases as the driving speed of the vehicle rises, so that the ratio of the slip falls.
When the torque transmitted to the tire is so high that the tire slips, the torque does not fully act to propel the vehicle, resulting in drawbacks that motive power is wasted, fuel efficiency drops, and the moving property of the vehicle is adversely affected.
Particularly when the fluctuation in the coefficient of friction is large or the coefficient of friction is very small, as on a muddy road, a partially icy road, a snowy road, a graveled road, or the like, not only does the stability of movement of the vehicle fall, but also it arises that the stopping distance increases in the case of locking of the wheel in braking, and it is even impossible to maintain the direction of movement of the vehicle in the case of locking of the rear wheel (in particular, in braking).
For the above-mentioned reasons, it has recently been desired that a four-wheel-drive vehicle, which was developed primarily for use on bad roads and in which the driving power of an engine is dividedly transmitted to four wheels to eliminate the above-mentioned drawbacks and problems, be adopted for use under various environmental conditions, including use on paved roads.
Since a rotation speed difference arises between the front and rear wheels of the four-wheel-drive vehicle due to the turning radius difference between the front and the rear wheels at the time of turning of the vehicle, torsional torque is caused (a tight corner braking phenomenon) between the drive shafts for the front and the rear wheels if the turning is performed on a high-friction-coefficient road (such as a paved road), on which the driving wheel and the surface of the road are less likely to slip relative to each other. For that reason, different types of four-wheel-drive vehicles have been developed in order to prevent the deterioration of the moving property of each vehicle due to the torsional torque, the increase in the wear of the tire, the shortening of the life of the vehicle, and so forth.
One of the different types of four-wheel-drive vehicles is a part time four-wheel-drive vehicle in which the driver shifts the four-wheel drive to the two-wheel drive when running on a high-friction-coefficient road such as a paved road. Another one of the different types of four-wheel-drive vehicles is a full time-four-wheel-drive vehicle equipped with a center differential unit for dividedly transmitting motive power to a front and a rear wheel drive shafts. The other and final one of the different types four-wheel-drive vehicles is a full time-four-wheel-drive vehicle in which the front or rear wheels are always driven and in which the rear or front wheels are driven through a viscous clutch which transmits torque by the viscosity of silicone oil or the like.
Although the part time-four-wheel-drive vehicle can be manufactured at a relatively low cost, not only is it troublesome to shift between the two-wheel drive and the four-wheel drive, but also it is likely that the vehicle is slowly turned due to the mistake of the driver despite being set in the state of four-wheel drive and that the driver needs to switch the two-wheel drive to the four-wheel drive according to his or her experience at the time of occurrence of a slip of the driving wheel during the two-wheel drive. It is less likely that every driver can precisely find out the occurrence of the slip of the driving wheel and take appropriate action.
Since the full time-four-wheel-drive vehicle equipped with the center differential unit has a front wheel drive differential unit, which dividedly transmits motive power to the right and left front wheels, a rear wheel drive differential unit, which dividedly transmits motive power to the right and left rear wheels, and the center differential unit, the vehicle has a problem that no motive power is transmitted to any of the remaining three of four driving wheels when one wheel is caused to spin or loses the tire grip due to overhanging on the road side or ditch, a slip on an icy road, or the like. For that reason, the center differential unit is provided with a differential locking mechanism for locking a differential means built in the unit. The differential locking mechanism is of the mechanical type or the electronic control type. In the mechanical type, a differential rotation which takes place in the center differential unit is stopped through manual shifting when no motive power is transmitted to the three of the four driving wheels in order to put the vehicle into the state of direct-connection four-wheel drive. In the electronic control type, the speed of the vehicle, the angle of turning of the vehicle, the racing of the drive shaft, and so forth are detected by sensors in order to put the differential locking mechanism into a locking or unlocking state through an electronic controller. As for the mechanical type, it is difficult to set a differential locking start time point, the time point cannot be changed depending on the moving condition of the vehicle, and it is more difficult to automate the differential locking mechanism. As for the electronic control type, a device for controlling the differential locking mechanism is so complicated that the cost of production of the mechanism is very high.
Since the center differential unit comprises an input shaft which receives motive power transmitted from an engine through a transmission, a differential case connected to the input shaft, a pinion shaft which is driven by the differential case, pinions rotatably attached to the peripheral surface of the pinion shaft, a first side gear which is engaged with the pinion and connected to a first differential means for driving the front or rear wheels, a second side gear which is engaged with the pinion and connected to a second differential means for driving the rear or front wheels, and the differential locking mechanism which engages the differential case and the side gear with each other through mechanical operation or electronic control, the cost of production of the center differential unit is very high and the weight of the vehicle is increased.
In the four-wheel-drive vehicle having the viscous clutch, the clutch transmits the torque by the viscosity of the silicone oil or the like. For that reason, the magnitude of the torque transmitted through the viscous clutch from a transmission is proportional to a relative rotation speed. As shown in FIG. 11, the transmitted torque T increases as a parabola convex upward, along with the increase in the relative rotation speed S, so that the transmitted torque T is relatively high even when the relative rotation speed S is low. For that reason, a slight tight corner braking phenomenon tends to occur at the time of turning of the vehicle. When the relative rotation speed S is high (on a bad road, a snowy road, or the like, the coefficient of friction on which is small), the transmitted torque T levels off so that the magnitude of the torque is not enough. As for a partially icy road, the response to the sharp change in the surface friction coefficient of the road tends to be late. If high torque is to be transmitted in order to achieve good driving performance, the viscous clutch needs to be enlarged, resulting in increasing the weight of the vehicle and restricting the degree of spatial freedom of installation of the viscous clutch. Once the size, form, and so forth of the viscous clutch are determined, the relation between the transmission speed and the slip speed is fixed so that the relation cannot be controlled from outside.